Generally, the manufacture of integrated circuits is based on various techniques such as the diffusion of impurities, the growth of silicon by epitaxy, and ion implantation. As illustrated schematically in FIG. 1, ion implantation is performed by an implantation machine 1 used for the introduction, by ion bombardment, into monocrystalline silicon wafers 2, of dopant agents 3 or dopant species, such as boron, phosphorus and arsenic. These dopant agents 3 modify the electrical properties of the semiconductor silicon and enable the obtaining of P type or N type doped zones 4. These small-sized doped zones 4 are defined during the implantation by an organic resin mask 5 deposited on the surface of the silicon wafers. The combination of small-sized P doped zones and N doped zones enables the integration, on a small surface area of silicon, of a large number of elementary electronic components forming a complex electronic circuit.
In recent years, the increase in integration density and in the rates of manufacture of the integrated circuits has gone hand-in-hand with the need for increasingly stringent controls over the processes of ion implantation. Thus, the main concern of the manufacturers of integrated circuits is to swiftly detect anomalies and drifts that might appear in the implantation machines to prevent wasting silicon wafers. Indeed, a delay of a few hours in the detection of an anomaly may lead to a loss of several tens or even several hundreds of silicon wafers, which corresponds to a considerable lost sum.
In practice, the main parameters to be controlled in an ion implantation process are: the incident energy of the ions expressed in keV and the dose expressed in ions/cm.sup.2. The energy determines the depth of penetration of the ions, in the range of some nanometers to some hundreds of nanometers. The dose, expressed in ions/cm.sup.2, is the number of ions per unit of surface area received by an implanted wafer. The dose depends on the duration of the implantation cycle which, in general is chosen to be as short as possible, and the ion flow-rate which may be set by a measurement of the ion current.
Although the implantation machines are fitted with various control machines and instruments, experience shows that the efficient setting of the energy and dose parameters cannot really be verified except with an analysis of the result obtained. Thus, various control and verification methods have been developed enabling the measurement of the characteristics of the doped zone and the correction, if necessary, of the initial settings. These control and verification methods make it necessary, in certain cases, to plan for the use of test wafers. Others can be applied directly to "product wafers", namely silicon wafers containing the integrated circuits in the process of being manufactured. However, to date, none of these methods is free of drawbacks.
Thus, the measurement of the resistivity per unit of surface area of the dosed zones by metal probe tips has the drawback of being destructive. In the context of large-scale production, this method leads to a waste of several tens of silicon wafers per week. Furthermore, this method offers only average sensitivity to the energy and in practice enables only a verification of the dose.
Another standard method known as "SIMS" (secondary ion mass spectrometry) includes an analysis of the doped zones by mass spectrometry. This method requires the preliminary abrasion of the parts to be analyzed and is therefore destructive. It is furthermore slow to implement, and remains reserved for laboratory applications.
Yet another method, known as THERMAWAVE, include analyzing the temperature of a doped zone excited by a laser beam. This method requires complex and costly instrumentation. There is no general consensus about its non-destructive character, as there are some who believe that the heating of the tested zone causes an annealing of the silicon.
There is also a known method commercially distributed by the firm IONSCAN. This method is presented in an article by Jack J. Cheng and Gary R. Tripp in "Solid State Technology", November 1983. This method relies on the observation that, in the presence of ion bombardment, the organic resins of the implantation masks undergo a phenomenon of carbonization that modifies their optical properties. This phenomenon is explained by the fact that the ion bombardment prompts the splitting of the polymer chains; the expulsion of the H, O, N species; and then the evaporation of the most volatile components. The remaining carbon atoms get recrystallized in the form of graphite. In practice, this method is implemented by a test wafer comprising a glass substrate that is transparent to ultraviolet rays and is coated with a film of organic resin. The test wafer is subjected to an ion implantation cycle and an IONSCAN apparatus is used to measure the variations in the optical density of the resin, giving information on the dose and the energy.
This method has the advantage of being non-destructive as it is possible to recycle the test wafer at will by the withdrawal of the carbonized resin and the deposit of a new organic film. However, it provides only mediocre sensitivity to the dose and energy. This is so especially at high doses where there is observed a smoothing of the fluctuations of the optical density of the resin owing to excessive carbonization. To get a clear idea, the sensitivity to the dose is 0.9 for phosphorus doses of about 10.sup.12 ions/cm.sup.2 and is only 0.1 for phosphorus doses of 10.sup.15 ions/cm.sup.2 (giving a variation of 10% in optical density for a 100% variation of the dose). Furthermore, the test wafer made of glass is not detected by the optical detectors of the automatic loading systems for implantation machines. As a result, this method requires intervention by hand. This entails risks of deterioration of the implantation machines, especially in the event of breakage of the glass wafer.
Finally, the article by Jan Vanhellemont and Philippe Roussel, "Characterization by Spectroscopic Ellipsometry of Buried Layer Structure in Silicon formed by Ions Beam Synthesis", Materials Science and Engineering, B12 (1992), pp. 165-172, proposes characterizing of the buried layers of a silicon wafer by spectroscopic ellipsometry. Ellipsometry has theoretical foundations which are fairly old. However, it has recently seen considerable advances owing to the devising and marketing of spectroscopic ellipsometers endowed with powerful software. This enables the computation of the thickness and refraction index of the layers of a multilayer thin-layer wafer through a wideband measurement of the cos.delta. and tan.psi. ellipsometric parameters of the wafer.
A software program of this kind executes a regression algorithm that relies on a structural model of a multilayer wafer that has to be given to the machine. In practice, the application of ellipsometry to the characterizing of the buried layers of a silicon wafer for the verification of an industrial process has a disadvantage in the need to prepare a structural model of the wafer. A structural model is not feasible because of its complexity resulting from the various buried layers that may be contained in an integrated circuit.
Ultimately, the conventional methods used in industry, as well as those that are at the experimental stage are entirely or partially beset by the drawbacks just referred to, namely that they:
are inapplicable to wafers produced, PA1 are destructive, PA1 require non-recyclable test wafers, PA1 require test wafers incompatible with the automatic loading systems of the implantation machines, PA1 provide mediocre sensitivity to the dose and energy parameters in a certain range of values, and/or PA1 are all far too slow or have a high cost of implementation.